Organotin maleates form a large family which has been well known for more than thirty years for their use in the heat stabilization of halogenated polymers, such as poly(vinyl chloride) (PVC) and chlorinated poly(vinyl chloride) (CPVC). Numerous references describe in particular organotin alkyl maleates, represented by the formula (R.sup.2).sub.2 Sn(OOC--CH.dbd.CH--COOR.sup.3).sub.2, where R.sup.2 represents a methyl, butyl, octyl or lauryl group and R.sup.3 a hydrocarbon chain containing from 1 to 12 carbon atoms and, more rarely, from 13 to 22 carbon atoms.
The effectiveness of organotin maleates in PVC compositions, in particular intended for articles for external use, is well known.
Thus, in the article entitled "Worldwide Weathering of Polyvinyl Chloride", by Emery Szabo and Robert Lally; Polymer and Engineering Science, April 1975, Vol. 15, No. 4, the results of long-term studies of exposure to weather are presented. The stabilizing compositions used were barium/cadmium soap compositions, organotin maleate compositions and organotin mercaptoacetate compositions. It is noted in the summary and in the conclusions that DBTM (dibutyltin maleate) gives the best results as UV stabilizer and that organotin mercaptoacetates give the worst results.
Although the superiority of compositions containing organotin maleates in the stabilization of PVC formulations subjected to weathering has been recognized for a long time, these compositions have not succeeded in fully satisfying market requirements because of many disadvantages.
Thus, British Patent 787,930 explains, from page 1, line 65 to page 2, line 3, that dibutyltin maleate is difficult to disperse and in addition generates, during the conversion of the PVC, a volatile maleic anhydride fraction which has lacrimogenic and irritating effects on the people handling it. In order to overcome these faults, GB 787,930 provides compositions containing organotin maleate hemiesters which are liquid, such as dibutyltin bis(monobutyl maleate). The presence of the alcohol group of the ester has the effect of decreasing the tin content, resulting in reduced heat stabilization, but does not remove the volatile fraction generated by the stabilization mechanism, that is to say the formation of the maleic acid hemiester by the reaction of the organotin bis(monoalkyl maleate) with the gaseous HCl generated during the processing of the PVC.
Another problem which is raised by the use of organotin maleates as heat stabilizer for vinyl chloride polymers is the difficulty of processing these formulations. Indeed, it is well known, in particular during the extrusion of rigid PVC, that the molten polymer containing the said stabilizers exhibits a strong tendency to adhere to the conversion equipment.
Combinations, sometimes complex, of lubricants are used to overcome this problem and, more rarely, attempts are made to modify the structure of the stabilizer itself.
Thus, Patent GB 1,378,851 provides for the addition of a mixture of paraffin wax and of a lubricating acrylic polymer to a PVC formulation containing dibutyltin di(methyl maleate), so as to reduce adhesion to the equipment. The improvement thus obtained is characterized by greater ease in detaching a film converted on a roll mill but remains limited, however.
The pronounced tendency towards adhesion of the organotin mlaleates conventionally used by PVC converters may also be reflected by the appearance of significant deposits in the conversion equipment and particularly at the inlet of the extrusion dies. Numerous publications and patents propose to solve this problem by combining various types of organic molecules with the organotin maleates.
Thus, Japanese Patent Application 61296048 demonstrates that the combination of a dibutyltin maleate with a metal soap containing a hydrocarbon chain which can range up to 29 carbon atoms, the metal being chosen from magnesium, calcium, strontium, barium or zinc, makes it possible to obtain PVC formulations exhibiting reduced plate-out.
In Japanese Patent Application 61209248, it is clearly demonstrated, during the extrusion of panels, that a substituted mercaptopropionic acid added to a formulation stabilized with dibutyltin di(ethylhexyl maleate) makes it possible to completely suppress plate-out.
It can also be seen, in Japanese Patent Application 04285651, that a combination of an orthophthalic acid hemiester comprising from 1 to 12 carbon atoms and of butyl stearate makes it possible to reduce the plate-out of a PVC formulation stabilized with dibutyltin maleate.
However, the significant, indeed excessive, addition of lubricants to PVC formulations, as is most often proposedo can exhibit disadvantages. Thus, an excessively high content of internal lubricants will have, inter alia, the consequences of increasing the plasticization of the PVC and thus of decreasing its Vicat point. In the case of rigid formulations, this can be a factor which prohibits the use of organotin maleates.
In the same way, an excess of external lubricants intended to limit problems of adhesion during extrusion will have a tendency to promote phenomena of exudation and thus of deposits on the equipment.
Thus, despite more than thirty years of research, heat stabilizers for PVC based on organotin maleates continue to have a limited use although they exhibit excellent stabilizing properties and very good resistance to weathering. Their properties cannot be regarded as entirely satisfactory:
either because, during processing, they decompose to give volatile products which are irritating and lacrimogenic, PA1 or because they are difficult to process, giving in particular formulations which have a strong tendency towards adhesion and which can cause problems of deposits on the conversion equipment. PA1 either an alcohol ROH in which R represents a linear or branched aliphatic hydrocarbon radical having a number of carbon atoms ranging from 23 to 50 or a mixture of saturated primary alcohols with a weight-average molecular mass Mw ranging from 340 to 718 and a polydispersity Mw/Mn in the region of 1 (Mn representing the number-average molecular mass), PA1 or an epoxyalkane C.sub.n H.sub.2n O in which n ranges from 23 to 50 or a mixture of epoxyalkanes with a weight-average molecular mass Mw ranging from 338 to 716; PA1 x is an integer equal to 1 or 2. PA1 an absorption band in the region of 1725 cm.sup.-1 characteristic of ester functional groups, PA1 an absorption band in the region of 1585 cm.sup.-1 characteristic of tin carboxylates, PA1 an absorption band in the region of 680 cm.sup.-1 characteristic of .tbd.Sn--O--Sn.tbd. bonds.